1. Field of the Invention
The invention relates generally to illumination systems for microlithographic projection exposure apparatus. More particularly, the invention relates to illumination systems comprising an optical field defining component that is positioned in or in close proximity to a pupil plane of the illumination system. Such a field defining component determines, together with stops, the geometry and the intensity distribution of a field that is illuminated by the illumination system on a reticle to be projected.
2. Description of Related Art
Microlithography (also called photolithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. More particularly, the process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to illumination light through a reticle (also referred to as a mask) in a projection exposure apparatus, such as a step-and-scan tool. The reticle contains a circuit pattern to be projected onto the photoresist. After exposure the photoresist is developed to produce an image corresponding to the circuit pattern contained in the reticle. Then an etch process transfers the circuit pattern into the thin film stacks on the wafer. Finally, the photoresist is removed.
A projection exposure apparatus typically includes an illumination system, a projection lens, a reticle alignment stage and a wafer alignment stage for aligning the reticle and the wafer, respectively. The illumination system illuminates a region of the reticle that may have the shape of an elongated rectangular slit. As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. For example, there is a need to illuminate the illuminated field on the reticle with a very uniform irradiance.
Another important property of illumination systems is the ability to manipulate the angular distribution of the illumination light bundle that is directed onto the reticle. In more sophisticated illumination systems it is possible to adapt the angular distribution of the illumination light to the kind of pattern to be projected onto the reticle. For example, relatively large sized features may require a different angular distribution than small sized features. The most commonly used angular distributions of illumination light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the intensity distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane, and thus there is only a small range of angles present in the angular distribution of the illumination light so that all light beams impinge obliquely with similar angles onto the reticle.
Since lasers are typically used as light sources in illumination systems, the illumination light bundle emitted by the light source has usually a small cross section and a low divergence. Therefore the geometrical optical flux, which is also referred to as the light conductance value, the Etendu-invariant or Lagrange-invariant, is small. Since the geometrical optical flux is not altered when a light bundle traverses an interface between media having different refractive indices, the geometrical optical flux cannot be changed by conventional refractive optical elements such as lenses.
Therefore most illumination systems contain optical elements that integrally increase the divergence of light passing the element. Optical elements having this property will in the following be generally referred to as optical raster elements. Such raster elements comprise a plurality of—usually periodically arranged—substructures, for example diffraction structures or microlenses.
From U.S. Pat. No. 6,285,443 an illumination system is known in which a first optical raster element is positioned in an object plane of an objective within the illumination system. A field defining component formed as a second optical raster element is positioned in an exit pupil plane of the objective. As a result of this arrangement, the first optical raster element determines the intensity distribution in the exit pupil plane and therefore modifies the angular distribution of light. At the same time the geometrical optical flux of the illumination light is increased. The field defining component modifies the size and geometry of the illuminated field on the reticle and also increases the geometrical optical flux of the illumination light bundle. Zoom optics and a pair of axicon elements allow to modify the intensity distribution in the pupil plane and therefore the angular distribution of the illumination light bundle.
Conventional illumination systems often comprise a glass rod or another light mixing element that generates a uniform irradiance in the reticle plane. However, it is difficult to preserve the polarization state of the illumination light with such light mixing elements. This is disadvantageous because it has been found out that illuminating the reticle with illumination light having a carefully selected polarization state may considerably improve the imaging of the reticle onto the photoresist.
For that reason illumination systems are designed that do not comprise light mixing elements such as glass rods. However, this requires that other means are found for achieving the desired uniform irradiance in the reticle plane. In step-and-scan tools in which the reticle is moved synchronously with the wafer during the projection, uniform irradiance in the direction perpendicular to the scan direction is of particular concern since the irradiance is not averaged by time integration as is the case in the scan direction.
One approach to solve this problem is to use an adjustable stop device as is disclosed in European patent application EP 0 952 491 A2. This device comprises two opposing rows of little adjacent blades that are arranged parallel to the scan direction. Each blade can be selectively inserted into the illumination light bundle. By adjusting the distance between the blades, the irradiance on the reticle can be manipulated in the direction perpendicular to the scan direction. However, is has been found out that using such a stop device alone does not meet the required accuracy with respect to the irradiance uniformity.
Another approach is to improve the field defining component that determines not only the geometrical shape, but has also a great impact on the intensity distribution in the reticle plane. Conventional field defining components are realized as diffractive optical elements or as refractive optical elements, for example micro-lens arrays.
Diffractive optical elements have the disadvantage that the zero's diffraction order cannot be sufficiently suppressed. As a result, the intensity distribution in the reticle plane comprises an array of bright spots. Apart from that, diffraction angles of more than about 18° require minimum feature sizes of the diffraction structures that can only be achieved by electron beam lithography. Blazed flanks of such minute diffraction structures have to be approximated by very few, for example 2, steps. This significantly reduces the diffraction efficiency of the device to values below 80%. In addition, the manufacture by electron beam lithography is a very slowly process so that these elements are extremely expensive.
Refractive optical elements, on the contrary, allow to introduce comparatively large angles. The main drawback of refractive optical elements, however, is the fact that the intensity distribution generated in the far field and thus in the reticle plane is not sufficiently uniform. Instead of being flat, the intensity distribution is characterized by a plurality of ripples that cannot be tolerated.
From WO 2005/015310 A2 an illumination system is known in which two different optical raster elements are positioned in two different pupil planes. One optical raster element increases the geometrical optical flux in a scan direction and the other optical raster element in a direction which is perpendicular to the scan direction.
U.S. Pat. No. 4,682,885 A discloses an illumination system with an optical integrator that includes four arrays of parallel cylinder lenses. Two arrays have cylinder lenses extending along the scan direction and having a first focal length. The other two arrays have cylinder lenses that extend perpendicular to the scan direction and have a second focal length that is larger than the first focal length.
U.S. Pat. No. 6,243,206 B1 discloses an illumination system in which a first microlens array is arranged in an object plane of an objective that comprises zoom optics and an axicon lens pair. The objective makes it possible to modify the irradiance distribution in its exit pupil and therefore the angular distribution of the illumination light bundle impinging on the mask. A second microlens array is arranged between the objective and a scattering element which is arranged in front of a honeycomb condenser (fly-eye optical integrator).
A similar illumination system is known from U.S. Pat. No. 6,583,937 B1. Here the second microlens array is dispensed with. The scattering element is arranged in the vicinity of an intermediate field plane.